High-energy solid formulations, such as propellants, explosives, and gasifiers, generally consist of particulate solids, such as fuel material, oxidizers, or both, held together by an elastomeric binder. These high-energy formulations also often include a plasticizer, such as a nitrate ester, which is a liquid prior to its incorporation into the formulation.
Organic compounds which contain nitrogen and fluorine are frequently used as fuel material and/or particulate oxidizers. These compounds are most suitable for making high-energy propellants because such compounds form gaseous HF as a decomposition product. During decomposition, the high heat of formation of HF is liberated to the surroundings, thereby doing mechanical work. Additionally, during the course of decomposition, fluorine is present as an oxidizing agent, and thus, no external source is required to complete oxidation.
Plasticizers are used in solid propellants and explosives to facilitate processing and increase flexibility and toughness, in addition to providing other benefits which vary with the nature and use of the formulation. Energetic or high-energetic plasticizers are those that provide energy in addition to flexibility and toughness, and their inclusion there/ore does not lessen the performance of the formulation. Considerations involved in the selection and use of plasticizers include compatibility with the other components of the formulation, including the primary energetic compounds and any binders present, the oxygen balance of the plasticizer, energy content, safety (i.e., stability with regard to detonation), and melting point. Plasticizers with melting points in a range which causes them to crystallize readily, for example, are of limited utility, since crystallization is detrimental to the plasticizer function and can adversely affect the mechanical properties of the propellant or explosive.
While the binder is an important means of maintaining the uniformity of the formulation and of holding it together, the binder material bums with substantially lower energy than the fuel. The binder thus imposes a limit on the energy content of the fuel material. One way of minimizing this limitation is to use a binder which release as much energy as possible when burning with the fuel. It is desirable, therefore, that the elastomeric binder have pendant groups which themselves are relatively high in energy.
Additionally, if a nitroester plasticizer is used in conjunction with the binder, nitroester-miscibility is required. Thus, in addition to being relatively high in energy, the polyethers and the elastomers formed therefrom should contain pendant groups which impart miscibility of the elastomer with the nitrate ester plasticizers. Nitro, nitrato, nitroamino and cyano groups are examples of pendant groups which impart nitrate ester-miscibility to the polymer and which have relatively high energies so as to contribute to the performance of the propellant.
Compounds containing two fluorine atoms bonded to nitrogen, i.e., a difluoroamino (NF.sub.2) group, have been extensively studied as ingredients for propellants and explosives. The difluoroamino group has higher energy, higher positive heat of formation and greater thermal stability than the other frequently used pendant groups (e.g. the nitrato group). In addition to significantly improved energy content, the difluoroamino group strongly enhances the performance of formulations containing boron and aluminum as fuels.
Practical use of the difluoroamino group in propellants and explosives has been limited, however. In compounds known to date, the difluoroamino group was found to impart unacceptably high impact sensitivity or chemical instability to the compound. Due to the strong electron-withdrawing nature of the difluoroamino group, NF.sub.2 -containing compounds have been found to be unstable and readily lose HF to form nitriles when alpha hydrogens are present. Therefore, the use of NF.sub.2 -containing compounds in explosives and propellants has been limited to those compounds containing difluoroamino groups on a tertiary center prepared from tetrafluorohydrazine, or geminal di-difluoroamino groups prepared from strong acid solutions of difluoroamine. In both cases the resulting products are shock sensitive and expensive to prepare.